1. Field of the Invention
This invention relates to glass-ceramic for information recording disks having a crystallized structure used suitably for substrate materials and the like for magnetic disk and magneto-Optic disk purposes and having improved surface characteristics, and besides the raw glass thereof being excellent in melting properties and mold ability.
2. Description of the Related Art
Recently, needs for magnetic disks and magneto-optic disks as external recording media used in main frame computers, personal computers and the like is increasing, and demands for high density recording is becoming stronger and the development therefor is proceeding rapidly.
In this connection, the following characteristics are required for, e.g. substrates used for magnetic disks:
(1) In order to improve recording density of magnetic disks, there is tendency to decreasing an amount of head floating so that a disk surface must be flat and smooth. PA1 (2) A degree of surface roughness (Ra) must be 10 Angstrom or less, and preferably from 3 to 8 Angstrom. PA1 (3) Substrate materials for magnetic disk should have no crystalline anisotropy, no defect, and further the texture of which must be dense, homogeneous, and fine. PA1 (4) The substrate materials must have such a mechanical strength and hardness sufficient for withstanding high-speed rotation of disks and contact with heads. PA1 (5) The glass must not have Na.sub.2 O ingredient exceeding a certain small amount, because when the glass raw material contains large amount of the Na.sub.2 O ingredient, Na+ion diffuses during a process of film formation so that the resulting glass deteriorates the film characteristics. PA1 (6) The glass must have chemical durability sufficient for withstanding washing or etching with various chemicals. PA1 (7) Melting and molding of the glass must be easy, and must be suitable for mass production.
An aluminum alloy has heretofore been used for magnetic disk substrate but an aluminum alloy substrate gives rise to projections or a spot-like unevenness in the polishing process due to various defects inherent in the material with the result that the aluminum alloy substrate is not sufficient in flatness and surface roughness for coping with high density recording.
For solving the problem of the aluminum substrate, various magnetic disk substrates such as chemically reinforced glass are known. However, chemically reinforced glass is instable in the reinforced layer when thinning of the disk is intended. Besides, a large quantity of Na.sub.2 O included in the glass adversely affects the film forming characteristic of the glass with resulting necessity for surface treatment.
Known in the art are various types of glass-ceramics as materials substituting for the aluminum alloy substrate and the chemically reinforced glass substrate For example, the SiO.sub.2 Li.sub.2 O system glass-ceramic disclosed in Japanese Patent Application Laid-open No. Sho 62-72547 contains lithium disilicate and lithium metasilicate as its predominant crystal phases. Since, however, the diameter of crystal grain is a large value of 3 .mu.m or over, the surface roughness (Ra) after polishing becomes a large value of 10 Angstroms or over which makes this glass ceramic unsuitable for a disk substrate for high density recording. Japanese Patent Application Laid-open No. Hei 2-247821 discloses a glass-ceramic which contains a crystal phase having a microstructure of a sheet silicate or a chain silicate. Since, however, the crystal grain structure of this glass-ceramic is acicular or rod-like. anisotropy of the crystal causes excessive holes and pits to be produced and therefore sufficient surface roughness cannot be obtained. Besides, the crystal of this glass-ceramic contains a large quantity of Na.sub.2 O ingredient so that dispersion of Na ion occurs in the film forming process which adversely affects the film characteristic.
One of the inventors of the present invention has disclosed in Japanese Patent Application Laid-open No. Hei 1-208343 a transparent glass-ceramic: of the SiO.sub.2 -Al.sub.2 O.sub.3 -ZnO-MgO-Pbo-TiO.sub.2 system which has transparency and high transient temperature and low-expansion characteristics and is suitable for use as substrates of various display, solar batteries and photo-masks. In this glass-ceramic, however, sufficient improvement in melting characteristic and devitrification characteristic of the base glass have not been made and therefore this glass-ceramic is disadvantageous in press molding of a disk which is useful for manufacturing a magnetic disk substrate and also disadvantageous in a large scale production in the disk forming process.